Process and apparatus for determining the presence,in a given area of the edge of a sheet or of a ribbon of transparent material

ABSTRACT

Method and apparatus for detecting whether either or both of two given zones is occupied by one or the other of two opposite edge portions of a transparent sheet or ribbon having a predetermined spatial orientation, the edge portions having a geometry fulfilling certain conditions, the method comprising the steps of supporting a sheet or ribbon with the aforesaid orientation, projecting a beam of light rays into the zones in a direction across and at an acute angle to one side of the supported sheet or ribbon and detecting the existence of light rays emergent from the zones deflected by edge portions therein and travelling across and at an acute angle to the other side of such supported sheet or ribbon; the apparatus comprising a support, a projector, and a detector for carrying out the aforesaid steps.

United States Patent [19] Jesse et al.

[451 Oct. 22, 1974 PROCESS AND APPARATUS FOR DETERMINING THE PRESENCE,IN A GIVEN AREA OF THE EDGE OF A SHEET OR OF A RIBBON OF TRANSPARENTMATERIAL [75] Inventors: Jean Josse, Fosses-la-Ville; Joseph Declaye,Moustier sur Sambre; Roger Albert, Chatelet; Marcel Bodart, Namur, allof Belgium [73] Assignee: Glaverbel-Mecaniver Chaussee de La Hulpe,Boitsfort, Belgium 22 Filed: Sept. 17,1973 21 Appl.No.: 397,93l

[30] Foreign Application Priority Data Oct. 3, 1972 Luxembourg 66220[52] US. Cl 356/199, 250/560, 250/561, 7 250/571, 356/160 [51] Int. CL.G0lb 11/04, GOlb ll/10,G01n 21/30 [58] Field of Search 356/199, 160;250/560, 250/561, 571

[56] References Cited UNITED STATES PATENTS 3,307,446 3/l967 Rottman250/560 3,495,089 2/1970 Brown 356/199 3,652,863 3/1972 Gaskell 250/571FOREIGN PATENTS OR APPLICATIONS l,l2l,l70 l/l962 Germany 250/560 PrimaryExaminer-James W. Lawrence Assistant Examiner-T. N. Grigsby Attorney,Agent, or Firm-Cushman, Darby & Cushman [5 7 1 ABSTRACT Method andapparatus for detecting whether either or both of two given zones isoccupied by one or the other of two opposite edge portions of atransparent sheet or ribbon having a predetermined spatial orientation,the edge portions having a geometry fulfilling certain conditions, themethod comprising the steps of supporting a sheet or ribbon with theaforesaid orientation, projecting a beam of light rays into the zones ina direction across and at an acute angle to one side of the supportedsheet or ribbon and detecting the existence of light rays emergent fromthe zones deflected by edge portions therein and travelling across andat an acute angle to the other side of such supported sheet or ribbon;the apparatus comprising a support, a projector, and a detector forcarrying out the aforesaid steps.

20 Claims, 5 Drawing [Figures PROCESS AND APPARATUS FOR DETERMINING THEPRESENCE, IN A GIVEN AREA OF THE EDGE OF A SHEET OR OF A RIBBON OFTRANSPARENT MATERIAL The present invention relates to a method ofdetecting whether a given zone is occupied by an edge portion of atransparent sheet or ribbon having a predetermined spatial orientation,said edge portion having a geometry fulfilling certain conditions, andto apparatus for use in performing such method.

The invention has particular, but not exclusive reference to detectingthe presence of such an edge portion of a glass sheet or ribbon in agiven zone during manufactureof the glass. This information isparticularly important for purposes of quality control.

It is known to project light beams along the boundaries of such a givenzone so that if one of such beams is obstructed by an opaque sheet edgeportion and the other is allowed to pass, then that edge portion isknown to be in the zone. This method is clearly not suitable for use inconnection with transparent sheet or ribbon edge portions, since bothsuch beams would be allowed to pass. whether or not an edge portion laywithin the zone.

It is an object of the invention to provide such a method whereby thepresence of such an edge portion in the given zone is detected withoutphysical contact with the sheet or ribbon edge, since such contact mightimpair the quality of the edge, and because of the risk of damage to thedetection apparatus should a sudden discontinuity in the edge beencountered.

The method of the present invention is therefore characterized in that abeam of light rays is projected into said zone in a direction such that,when a sheet or ribbon having the aforesaid predetermined orientationand edge portion of said geometry is present in said zone, the lightrays enter the edge portion after travelling in a direction across andat an acute angle to one side of the sheet or ribbon and light raysemerge from said edge portion to travel in a direction across and at anacute angle to the other side of said sheet or ribbon after deflectionby said edge portion, and in that the existenceof rays which thus emergeis detected.

The method according to the present invention has the advantage that itenables such detection in respect of transparent sheets or ribbons to beperformed rapidly and accurately without physical contact with the edgeportion if present. Also, the required detection may take place atpositions well spaced from the sheet or ribbon, e.g., at a distance ofseveral meters therefrom. The method can be applied in situations wherethe sheet or ribbon is very hot without overheating the apparatus usedin performing the method. For example, the method may be applied on acontrol factor in the manufacture of flat glass, e.g., in the annealinglehr of a glass drawing machine.

Preferably, the light beam is caused to scan the zone, so that regularindications of the presence of such an edge portion in said zone may beobtained.

The method according to the invention also contemplates detectingwhether either or both of two given zones is or are occupied by one orthe other of two opposite edge portions of a transparent sheet or ribbonhaving a predetermined spatial orientation, such edge portions havinggeometries fulfilling certain conditions. This detection is achieved bycausing the light beam to scan both said zones. In this way, informationabout the presence of the opposite edge portions of the sheet or ribboncan be obtained without duplicating the light beam projecting apparatuswhich is required.

Arrangements where the method is applied for detecting whether either orboth of two given zones is or are occupied by one or the other of twoopposite edge portions of a transparent sheet or ribbon having apredetermined orientation, such edge portions having geometriesfulfilling certain conditions, and the light rays which are projectedinto both said zones are emitted from a common source and the deflectedrays emergent from both said zones are detected at a single position,have the advantage that information about the presence of the oppositeedge portions of the sheet or ribbon can be obtained without duplicatingany light ray detecting apparatus which is required.

Preferably, light rays which would otherwise pass straight to thedetection position through a part of the sheet or ribbon located betweensaid edge portions are screened off so that detection of the deflectedrays, if any, is not disturbed by the projected beam.

When said light beam is displaced without interruption for scanning bothsaid zones in succession and the time interval between successivedetections of deflected light rays is measured, the width of the sheetor Preferably, said beam of light is caused to scan at least one saidzone and a reference signal is generated at at least one predeterminedpoint in such scanning movement, and the time interval betweengeneration of said reference signal and detection of the existence oflight rays deflected by an associated edge portion is measured.

It is advantageous if the method is continuously applied while a sheetor ribbon is in motion along a path comprising said zone or zones, sinceany changes in the position of one or both edges of the sheet or ribboncan be immediately indicated.

Apparatus according to the invention for detecting whether a given zoneis occupied by an edge portion of a transparent sheet or ribbon having apredetermined spatial orientation, said edge portion having a geometryfulfilling certain conditions, preferably comprises a support forsupporting such sheet or ribbon with said orientation, a projector forprojecting a beam of light rays into said zone in a direction across andat an acute angle to one side of a sheet or ribbon having saidorientation, and a detector arranged to detect the existence of lightrays emergent from said zone deflected by an edge portion and travellingacross and at an acute angle to the other side of such sheet or ribbon.

This apparatus has many advantages. It is extremely simple, and may becontained wholly within the projected plan areaof the sheet orcribbon,thus saving space. The apparatus is readily adaptable to detecting thepresence of a sheet or ribbon edge portion in a large variety of givenzones merely by pointing the projector and detector in the appropriatedirections, without otherwise moving them.

Preferably, said projector comprises means operative to cause said lightbeam to scan said zone. The specific advantages of this and otherpreferred features of apparatus according to the invention will readilybe appreciated from the advantages set forth above in relation tocorresponding preferred features of the method according to theinvention.

Preferred embodiments of apparatus according to the invention, fordetecting whether either or both of two given zones is or are occupiedby one or the other of two opposite edge portions of a transparent sheetor ribbon having a predetermined spatial orientation, such edge portionshaving geometries fulfilling certain conditions, are characterized inthat said projector comprises means operative to cause said light beamto scan both said zones.

Advantageously, said projector is operative to project light rays intoboth said zones, and said detector is arranged to detect the existanceof deflected light rays emergent from both said zones.

Preferably the apparatus comprises a screen located between theprojector and the detector.

Embodiments wherein said projector is operative to cause said light beamto scan both said zones in succession without interruption and saiddetector comprises timing means for measuring the time interval betweensuccessive detections of deflected light rays, are preferred since theyenable the width of the sheet or ribbon to be measured.

Advantageously, said projector comprises means operative to cause saidlight beam to scan at least one said zone and the apparatus furthercomprises a generator for generating a reference signal at at least onepredetermined point in such scanning movement, there being furtherprovided means for measuring the time interval between generation ofsuch reference signal and detection of the existence of light raysdeflected by a said edge portion.

Preferably, said support is a conveyor, and the apparatus is adapted tooperate continuously during movement of a sheet or ribbon along theconveyor.

. Various embodiments of the invention will now be described by way ofexample with reference to the accompanying diagrammatic drawings, inwhich:

FIGS. 1 and 2 illustrate the course of a beam of light in the zone of anedge portion of a transparent sheet having a plane edge and a curvededge respectively;

FIG. 3 illustrates one embodiment of apparatus for the performance of amethod according to the invention;

FIG. 4 shows another embodiment of apparatus according to the invention;and

FIG. 5 illustrates a further embodiment of apparatus according to theinvention.

FIG. 1 shows a glass sheet 1 having a rectilinear edge face 2 lying atright angles to the main, plane of the sheet 1. In FIG. 2 there is showna glass ribbon 3 having an edge face 4 which is rounded off in the shapeof a semi-circle and whose median perpendicular coincides with the mainplane of the ribbon 3.

In the case of both the sheet I and the ribbon 3, when an incident rayof light 5 enters the glass it is deflected from its previous directionof propagation by refraction and passes inside the glass as far as therectilinear edge face 2 of the sheet 1 or the curved edge face 4 of theribbon 3. The ray of light 5 is substantially completely reflected backinto the glass at the edge faces 2 and 4, and the deflected ray of light6 passes to the next boundary between the glass and its surroundings andis there again deflected from its direction of propagation in the glassby refraction.

The existence of the deflected ray of light 6 is detected by theperformance of the method according to the invention. The deflected rayof light 6 is strongly deflected in relation to the incident ray oflight 5 can be detected without the risk of disturbance by the incidentbeam of light.

FIG. 3 shows an apparatus according to the invention for detectingwhether eachof two given zones 7 is occupied by one or the other of twoopposite edge portions 8, 9 of a ribbon 10 of glass in an annealing Iehr11. The glass ribbon 10 moves substantially in the main median plane ofthe annealing lehr ll perpendicularly to the plane of the drawing. Theannealing lehr 11 has a bottom 12, side walls 13, and a roof 14. Thebottom 12 and roof 14 of the annealing lehr 11 have lighttransmittingregions 15 and 16 respectively through which an incident beam of light 5and also deflected beams 6, 26 of light can pass. Lying in the mid-planel7 perpendicular to the glass ribbon l0 and below the ribbon is a rotarymirror 18 analogous to that of a mirror galvanometer located at atransmitting point 19 on the path of a beam of light emitted by a laser20. This beam of light 5 is caused to scan by the rotation of the rotarymirror 18 over a zone which extends to either side beyond the edges 8, 9of the ribbon 10.

The incident beam of light 5 is reflected and refracted at the innersurfaces of each of the edges 8, 9 during the course of scanning and thedeflected beams of light 6, 26 pass back towards the vertical plane 17and form with the ribbon 10 angles substantially equal to the angleformed by the incident beam of light 5 with the ribbon 10 at thoseedges.

A photo electric detector 21 which receives the deflected beams of light6, 26 at the moments when they pass through reference points 22, is alsoshown in the vertical plane 17 of the ribbon l0 and on reception of adeflected beam of light, this detector delivers an electrical impulse.

The detector 21 is connected to a timing device 23 which measures theinterval of time between successive pulses. Connected to the timingdevice 23 is an indicating apparatus 24 which constantly indicates themeasuring results. If the rate of rotation of the mirror 18 is known,the speed of scanning is known, and hence the indicating apparatus 24may be calibrated to give a direct reading of the width of the ribbon10.

Disposed between the glass ribbon l0 and the detector 21 is a screen 25so wide that when the beam of light 5 is caused to scan, only the beamsof light 6, 26 reflected by the edges 8, 9 can reach the detector 21,while the incident beam of light 5 passing through the rest of the glassribbon 10 during intermediate portions of the scanning period, isscreened from the detector. The screen 25 can be omitted if the roof 14and/or the bottom 12 of the annealing Iehr 11 have a central portionwhich is opaque and adequately screens the detector against the incidentbeam of light 5 during this period of the scanning.

The rotary mirror 18 is preferably pivoted backwards and forwards atconstant speed, so that the incident beam of light 5 scans the ribbon 10at a constant frequency. As soon as the beam oflight 5 impinges on theedge 8, it is deflected there and the beam of light 6 deflected by theedge 8 passes to the detector 21 where a pulse is produced which startsthe timing device 23. As soon as the incident beam of light 5 arrivesduring its scan at the opposite edge 9, it is again deflected and thebeam of light 26 deflected by the edge 9 also passes to the detector 21and produces there a second pulse which stops the timing device 23. Theinterval between the two pulses is a measurement of the width of theglass ribbon 10, since the distance apart of the edges 8, 9 can bedetermined from the measured interval in time and the known speed ofrotation of the rotary mirror 18 in conjunction with its distance fromthe main plane of the glass ribbon 10.

The apparatus illustrated in FIG. 3 produces a pulse at its detector 21when the beam of light 5 is deflected at either edge 8, 9 of the ribbon10, provided that the respective edge is located in one of the givenzones 7. The position and extent of each of these zones 7 are determinedby the position and extent of the associated transparent regions l5, 16of the annealing lehr 11, and of the screen 25. In order that a pulseshould be produced at the detector 21, it is also necessary that therespective edge 8 or 9 of the glass ribbon 10 should have a geometrywhich will cause deflection of the beam to the required degree. Certainlenticular and prismatic shapes will not give rise to this deflection,but these shapes are not encountered in practice in a ribbon travellingalong an annealing lehr.

In H6. 4 like elements have like references to those in FIG. 3, but onlythat part of the measuring apparatus which is associated with the edge 9is shown.

Shown in the drawing above the main plane of the glass ribbon 10 is areference detector 27, arranged to receive light passing through areference point 28 disposed at such a distance from the vertical plane17 that the beam of light 5 transmitted from the transmitting point 19on the rotary mirror 18 impinges on the reference detector 27 withoutpassing through the glass ribbon 10. Reception of this beam causesreference detector 27 to transmit a pulse to the timing device 23 tostart the time measurement therein. In scanning from the reference point27 to the edge of the ribbon 10, the incident beam of light 5 sweeps anangle alpha. As soon as the beam of light 5 has swept this angle alpha,it is deflected at the edge 9 as described above, and'the beam of light26 deflected by the edge 9 is received by the detector 21 which thentransmits a pulse to the timing device 23 and terminates the timemeasurement. The interval in time measured by the timing device 23between the pulse from the reference detector 27 and the pulse from themeasuring detector 21 is a measurement of the value of the angle alpha,since both the distance of the transmitting point 19 on the on therotary mirror 18 from the glass ribbon 10 and also the speed of scanningare known. Since the distance of the reference point 28 from the mainplane of the glass ribbon 10 and from the vertical plane 17, which isalso the median vertical plane of the annealing lehr 11 are also known,the exact position of the edge 9 inside the annealing lehr is given bythe value of the angle alpha.

lf with the apparatus illustrated in FIG. 4 the distance of the edge 9from the edge 8 is also measured, the same measurement can varyaccurately determine not only the presence of a glass ribbon 10 in givenzones 7 of the annealing lehr 11 and the width of the glass ribbon 10,but also the position of the glass ribbon 10 inside the annealing lehr.If the beam of light 5 periodically scans between the reference point 28and the edge 8, the position of the glass ribbon l0 inside the annealinglehr 11 can be constantly monitored and indicated by the indicatingdevice 24.

Lastly, in the construction illustrated in FIG. 5, the laser lightsource 20 is disposed on a device 29 which can be pivoted around a point30 analogous with the fixed transmitting point 19 of FIGS. 3 and 4.

In the apparatus illustrated in FIG. 5 a reference point is given by acontact (not shown) which the pivoting device 29 actuates at one limitof its pivoting movement. As soon as the contact is made, a pulse istransmitted to the timing device 24 (not shown in FIG. 5). When the beamof light 5 has been pivoted through the angle alpha and impinges on theedge 9, it is deflected there and passes in the form of a deflected beamof light 26 to the measuring detector 21 which, on detection of the beamof light 26, transmits a pulse to the timing device to terminate timemeasurement. The measured interval in time between the pulse from thecontact and the pulse from the measuring detector 21 is again themeasurement of the angle alpha swept by the beam of light from thereference point to the edge 9. Once again, as in the cases describedhereinbefore, the angle alpha is a measurement of the position of theedge 9 of the glass ribbon 10 in relation to the reference point, (heregiven by the contact), since the distances between the pivoting point 30of the pivoting device 29 and the vertical axis of the annealing lehr 11and also the main plane of the glass ribbon 10 are known.

It thus will be seen that the objects of this invention have been fullyand effectively accomplished. It will be realized, however, that theforegoing preferred specific embodiment has been shown anddescribed forthe purpose of illustrating the functional and structural principles ofthis invention and is subject to change without departure from suchprinciples. Therefore, this invention includes all modificationsencompassed within the spirit and scope of the following claims.

What is claimed is:

l. A method of detecting whether a given zone is occupied by an edgeportion of a transparent sheet or ribbon having a predetermined spatialorientation, said edge portion having a geometry fulfilling certainconditions, said method comprising the steps of projecting a beam oflight rays into said zone in a direction such that, when a sheet orribbon having said predetermined orientation and an edge portion of asaid geometry is present with its edge portion in said zone, the lightrays enter the edge portion after travelling in a direction across andat an acute angle to one side of the sheet or ribbon and light raysemerge from said edge portion to travel in a direction across and at anacute angle to the other side of said sheet or ribbon. after deflectionby said edge portion, and detecting the existence of rays which thusemerge after deflection by said edge portion.

2. A method according to claim 1, wherein said light beam is displacedto scan said zone.

3. A method according to claim 2, wherein said beam of light rays isprojected into a second zone during the scanning displacement in adirection such that, when said sheet or ribbon having said predeterminedorientation and an opposite edge portion of said geometry is presentwith its opposite edge portion in said second zone, the light rays enterthe opposite edge portion after travelling in a direction across and atan acute angle to one side of the sheet or ribbon and light rays emergefrom said opposite edge portion to travel in a direction across and atan acute angle to the other side of said sheet or ribbon afterdeflection by said other edge portion, and the existence of rays whichthus emerge after deflection by said other edge portion is detected.

4. A method according to claim 3, wherein the light rays emergent afterdeflection by both of said edge portions are detected at a singleposition.

5. A method according to claim 4, wherein the light rays which wouldotherwise pass straight to the detection position through a part of thesheet or ribbon located between said edge portions are screened off.

6. A method according to claim 5, wherein said light beam is displacedwithout interruption to effect the scanning of both said zones insuccession and the time interval between successive detections ofemergent light rays is measured.

7. A method according to claim 6, wherein said sheet or ribbon iscontinuously moved along a path in which said zones are located duringsaid projecting, detecting, and measuring steps.

8. A method according to claim 5, wherein a reference signal isgenerated at at least one predetermined point in the scanning beam ofsaid light displacement and the time interval between the generation ofsaid reference signal and the detection of the existence of emergentlight rays deflected by an associated edge portion is measured.

9. A method according to claim 8, wherein said sheet or ribbon iscontinuously moved along a path in which said zones are located duringsaid projecting, detecting. and measuring steps.

10. A method according to claim 3, wherein said sheet or ribbon iscontinuously moved along a path in which said zones are located duringsaid projecting and detecting steps.

11. A method according to claim 1, wherein said sheet or ribbon iscontinuously moved along a path in which said zone is located duringsaid projecting and detecting steps.

12. Apparatus for detecting whether a given zone is occupied by an edgeportion of a transparent sheet or ribbon having a predetermined spatialorientation, said edge portion having a geometry fulfilling certainconditions, said apparatus comprising support means for supporting asheet or ribbon with said orientation, projector means for projecting abeam of light rays into said zone in a direction across and at an acuteangle to one side of a sheet or ribbon having said orientation supportedby said support means, and detector means for detecting the existence oflight rays emergent from said zone deflected by an edge portion thereinand travelling across and at an acute angle to the other side of thesheet of ribbon supported by said support means.

13. Apparatus according to claim 12 wherein said projector meansincludes means for causing said light beam to scan said zone.

14. Apparatus according to claim 13 wherein said support means comprisesa conveyor for continuously moving a sheet or ribbon along a path inwhich said zone is located.

15. Apparatus for detecting whether either or both of two given zones isoccupied by one or the other of two opposite edge portions of atransparent sheet or ribbon having a predetermined spatial orientation,said edge portions having a geometry fulfilling orientation, said edgeportions having a support means for supporting a sheet or ribbon withsaid orientation, projector means for projecting a beam of light raysinto said zones in a direction across and at an acute angle to one sideof a sheet or ribbon having said orientation supported by said supportmeans and detector means for detecting the existence of light raysemergent from said zones deflected by edge portions therein andtravelling across and at an acute angle to the other side of such sheetor ribbon supported by said support means.

16. Apparatus according to claim 15, wherein said projector meansincludes means for causing said light beam to scan said zones.

17. Apparatus according to claim 16, including a screen located betweensaid projector means and said detector means.

18. Apparatus according to claim 17, wherein said scanning means isoperable to cause said light beam to scan both said zones in successionwithout interruption and said detector means includes means formeasuring the time interval between successive detections of deflectedlight rays.

19. Apparatus according to claim 17, including generator means forgenerating a reference signal at at least one predetermined point in thescanning movement of said light beams, said detector means includingmeans for measuring the time interval between the generation of suchreference signal and detection of the existence of light rays deflectedby an associated edge portion.

20. Apparatus according to claim 19, wherein said support means is aconveyor for continuously moving a sheet or ribbon along a path in whichsaid zones are located.

1. A method of detecting whether a given zone is occupied by an edgeportion of a transparent sheet or ribbon having a predetermined spatialorientation, said edge portion having a geometry fulfilling certainconditions, said method comprising the steps of projecting a beam oflight rays into said zone in a direction such that, when a sheet orribbon having said predetermined orientation and an edge portion of asaid geometry is present with its edge portion in said zone, the lightrays enter the edge portion after travelling in a direction across andat an acute angle to one side of the sheet or ribbon and light raysemerge from said edge portion to travel in a direction across and at anacute angle to the other side of said sheet or ribbon after deflectionby said edge portion, and detecting the existence of rays which thusemerge after deflection by said edge portion.
 2. A method according toclaim 1, wherein said light beam is displaced to scan said zone.
 3. Amethod according to claim 2, wherein said beam of light rays isprojected into a second zone during the scanning displacement in adirection such that, when said sheet or ribbon having said predeterminedorientation and an opposite edge portion of said geometry is presentwith its opposite edge portion in said second zone, the light rays enterthe opposite edge portion after travelling in a direction across and atan acute angle to one side of the sheet or ribbon and light rays emergefrom said opposite edge portion to travel in a direction across and atan acute angle to the other side of said sheet or ribbon afterdeflection by said other edge portion, and the existence of rays whichthus emerge after deflection by said other edge portion is detected. 4.A method according to claim 3, wherein the light rays emergent afterdeflection by both of said edge portions are detected at a singleposition.
 5. A method according to claim 4, wherein the light rays whichwould otherwise pass straight to the detection position through a partof the sheet or ribbon located between said edge portions are screenedoff.
 6. A method according to claim 5, wherein said light beam isdisplaced without interruption to effect the scanning of both said zonesin succession and the time interval between successive detections ofemergent light rays is measured.
 7. A method according to claim 6,wherein said sheet or ribbon is continuously moved along a path in whichsaid zones are located during said projecting, detecting, and measuringsteps.
 8. A method according to claim 5, wherein a reference signal isgenerated at at least one predetermined point in the scanning beam ofsaid light displacement and the time interval between the generation ofsaid reference signal and the detection of the existence of emergentlight rays deflected by an associated edge portion is measured.
 9. Amethod according to claim 8, wherein said sheet or ribbon iscontinuously moved along a path in which said zones are located duringsaid projecting, detecting. and measuring steps.
 10. A method accordingto claim 3, wherein said sheet or ribbon is continuously moved along apath in which said zones are located during said projecting anddetecting steps.
 11. A method according to claim 1, wherein said sheetor ribbon is continuously moved along a path in which said zone islocated during said projecting and detecting steps.
 12. AppaRatus fordetecting whether a given zone is occupied by an edge portion of atransparent sheet or ribbon having a predetermined spatial orientation,said edge portion having a geometry fulfilling certain conditions, saidapparatus comprising support means for supporting a sheet or ribbon withsaid orientation, projector means for projecting a beam of light raysinto said zone in a direction across and at an acute angle to one sideof a sheet or ribbon having said orientation supported by said supportmeans, and detector means for detecting the existence of light raysemergent from said zone deflected by an edge portion therein andtravelling across and at an acute angle to the other side of the sheetof ribbon supported by said support means.
 13. Apparatus according toclaim 12 wherein said projector means includes means for causing saidlight beam to scan said zone.
 14. Apparatus according to claim 13wherein said support means comprises a conveyor for continuously movinga sheet or ribbon along a path in which said zone is located. 15.Apparatus for detecting whether either or both of two given zones isoccupied by one or the other of two opposite edge portions of atransparent sheet or ribbon having a predetermined spatial orientation,said edge portions having a geometry fulfilling orientation, said edgeportions having a support means for supporting a sheet or ribbon withsaid orientation, projector means for projecting a beam of light raysinto said zones in a direction across and at an acute angle to one sideof a sheet or ribbon having said orientation supported by said supportmeans and detector means for detecting the existence of light raysemergent from said zones deflected by edge portions therein andtravelling across and at an acute angle to the other side of such sheetor ribbon supported by said support means.
 16. Apparatus according toclaim 15, wherein said projector means includes means for causing saidlight beam to scan said zones.
 17. Apparatus according to claim 16,including a screen located between said projector means and saiddetector means.
 18. Apparatus according to claim 17, wherein saidscanning means is operable to cause said light beam to scan both saidzones in succession without interruption and said detector meansincludes means for measuring the time interval between successivedetections of deflected light rays.
 19. Apparatus according to claim 17,including generator means for generating a reference signal at at leastone predetermined point in the scanning movement of said light beams,said detector means including means for measuring the time intervalbetween the generation of such reference signal and detection of theexistence of light rays deflected by an associated edge portion. 20.Apparatus according to claim 19, wherein said support means is aconveyor for continuously moving a sheet or ribbon along a path in whichsaid zones are located.